[Paper Review] A CEERS Discovery of an Accreting Supermassive Black Hole 570 Myr after the Big Bang: Identifying a Progenitor of Massive z > 6 Quasars

Rebecca L. Larson, Steven L. Finkelstein+50 more|arXiv (Cornell University)|Mar 15, 2023

Astrophysical Phenomena and Observations21 citations

TL;DR

The paper reports JWST discoveries of an accreting supermassive black hole at z = 8.679 in CEERS_1019, with emission-line analysis and implications for black hole seeding and growth in the early universe.

ABSTRACT

We report the discovery of an accreting supermassive black hole at z=8.679, in CEERS_1019, a galaxy previously discovered via a Ly$α$-break by Hubble and with a Ly$α$ redshift from Keck. As part of the Cosmic Evolution Early Release Science (CEERS) survey, we observed this source with JWST/NIRSpec spectroscopy, MIRI and NIRCam imaging, and NIRCam/WFSS slitless spectroscopy. The NIRSpec spectra uncover many emission lines, and the strong [O III] emission line confirms the ground-based Ly$α$ redshift. We detect a significant broad (FWHM~1200 km/s) component in the H$β$ emission line, which we conclude originates in the broad-line region of an active galactic nucleus (AGN), as the lack of a broad component in the forbidden lines rejects an outflow origin. This hypothesis is supported by the presence of high-ionization lines, as well as a spatial point-source component embedded within a smoother surface brightness profile. The mass of the black hole is log($M_{BH}/M_{\odot})=6.95{\pm}0.37$, and we estimate that it is accreting at 1.2 ($\pm$0.5) x the Eddington limit. The 1-8 $μ$m photometric spectral energy distribution (SED) from NIRCam and MIRI shows a continuum dominated by starlight and constrains the host galaxy to be massive (log M/M$_{\odot}$~9.5) and highly star-forming (SFR~30 M$_{\odot}$ yr$^{-1}$). Ratios of the strong emission lines show that the gas in this galaxy is metal-poor (Z/Z$_{\odot}$~0.1), dense (n$_{e}$~10$^{3}$ cm$^{-3}$), and highly ionized (log U~-2.1), consistent with the general galaxy population observed with JWST at high redshifts. We use this presently highest-redshift AGN discovery to place constraints on black hole seeding models and find that a combination of either super-Eddington accretion from stellar seeds or Eddington accretion from massive black hole seeds is required to form this object by the observed epoch.

Motivation & Objective

• Motivate constraints on black hole seeding and early growth mechanisms during the Epoch of Reionization.
• Identify and characterize an accreting SMBH in a z>8 galaxy using JWST spectroscopy.
• Infer host galaxy properties and gas conditions to inform BH-galaxy co-evolution models.

Proposed method

• JWST CEERS observations using NIRSpec, NIRCam, MIRI, and WFSS to obtain spectroscopy and multi-band imaging.
• Emission-line fitting via a Gaussian plus continuum model with MCMC (10,000 iterations, 100 walkers) to determine line fluxes and uncertainties.
• Continuum fitting by masking lines, smoothing, and cross-checking with photometric SEDs to derive line significance.
• Slit-loss corrections based on NIRCam morphology and aperture matching to photometry.
• Combine G140M, G235M, G395M spectra into a single high-S/N spectrum for line measurements.

Experimental results

Research questions

• RQ1 What is the redshift and nature of CEERS_1019 (is it an AGN at z>8) as revealed by JWST spectroscopy?
• RQ2 What are the emission-line characteristics (e.g., [O III], Lyα, high-ionization lines) and what do they imply about the ionizing source?
• RQ3 What are the host-galaxy properties (stellar mass, SFR, metallicity) and gas conditions (ne, U) of this system?
• RQ4 What BH mass and accretion rate are inferred, and how do these constraints inform BH seeding and growth models at z>8?

Key findings

• CEERS_1019 is an accreting SMBH at z = 8.679 with a broad Hβ component (FWHM ~ 1200 km s−1) and high-ionization lines (N V, N IV], C III]).
• Black hole mass is log(MBH/M⊙) = 6.95 ± 0.37, accreting at 1.2 ± 0.5 times the Eddington limit.
• Host galaxy is massive with log(M/M⊙) ~ 9.5, SFR ~ 30 M⊙ yr−1, and sSFR ~ −7.9 Gyr−1; gas is metal-poor (Z/Z⊙ ~ 0.1), dense (ne ~ 10^3 cm−3), and highly ionized (log U ~ −2.1).
• The discovery provides constraints on BH seeding, requiring either super-Eddington accretion from stellar seeds or Eddington accretion from very massive seeds to reach the observed mass by z>8.
• Emission-line analysis used JWST/NIRSpec M-gratings to detect Lyα, NV, NIV], [O III], Ne III, Hδ, and other lines, with 1D extraction and careful error calibration.
• This z>8 AGN represents a progenitor population informing the formation pathways of massive z>6 quasars.